1. Field of the Invention
The present invention relates to wireless telecommunications, and in particular, to a method of providing dispatch mode calling in a code division multiple access wireless network.
2. Description of the Prior Art
"Dispatch mode" is a radio communications technique where one radio communicates to many other radios using short bursts of communications. Many groups such as taxi drivers and police officers desire to communicate with dispatch mode radio devices because it allows them to speak with a number of other people at the same time without initiating a plurality of separate phone calls. Until recently, dispatch mode services were only available in two-way radio systems such as walkie talkies. Although these radio systems work effectively when all of the users are located close together, they become ineffective once the users travel beyond the range of the radio transceivers.
Nextel has recently begun offering dispatch mode operations in its enhanced specialized mobile radio (ESMR) systems. ESMR can be deployed on a cellular network and, therefore, supports network features that enable users to operate in dispatch mode over greater distances. ESMR systems use time division multiple access (TDMA) technology which allocates a discrete amount of frequency band width to each user of the system to permit many simultaneous conversations. Each caller is allowed to transmit in predetermined time slots so that channelization of users in the same band is achieved through separation in time. Unfortunately, the capacity of these networks is limited by the number of available time slots. TDMA networks also suffer from poor call clarity, "hard" call hand-offs and minimal call security.
Companies such as Sprint PCS have recently begun offering wireless communication services using code division multiple access (CDMA) techniques. CDMA is a digital spread-spectrum modulation technique which digitizes wireless conversations and tags them with special codes. The digitized data is spread across the frequency band in a pseudo random pattern. Receiving mobile phones are instructed to decipher only the data corresponding to particular codes to reconstruct the signal. CDMA networks offer advantages over TDMA networks including increased network capacity, fewer dropped calls because of better hand-off methods, improved voice clarity, improved privacy and transmission security, and enhanced services such as text messaging and data transmissions. However, until now, CDMA wireless networks have not included dispatch mode calling capabilities.